Many adjectives have degrees of grade or intensity: big (adjective), bigger (comparative adjective), and biggest (superlative adjective), for examples of varying grades, or loud (adjective), louder (comparative adjective), and loudest (superlative adjective) as various levels of intensity. Others, however, have no variation: You cannot (notwithstanding the poetic license of the US Constitution’s “a more perfect union”) be perfecter than someone else or be the perfectest of all.
Such terms, classified as nongradable adjectives, are called absolutes: Just as one cannot be the perfectest person, one cannot be very unique or more correct, or the most unique or correct.
Despite the definitive term absolute, however, there is a little wiggle room: When absolutes become modifiers or are themselves otherwise modified, the rules are relaxed: Someone can be more uniquely situated than someone else, or more politically correct.
Likewise, terms that seem absolute — something can’t be more excellent or more impossible than sometime else — aren’t necessarily so: You can say that an experience was quite excellent or that a task was nearly impossible. (These, however, are qualitative, not quantitative, grades. You can measure that something is hotter than something else, but you can’t quantify excellence or possibility. Probability, yes; possibility, no.)
Other absolutes include references to states of being, as with alive and dead and white and black, words that express extremes of size such as gigantic and minuscule, terms that refer to polar opposites of quality, like terrible and terrific, and those that indicate outliers of emotion: furious, overjoyed, distraught.
Some words that can be used in the same situations are not necessarily interchangeable: For example, as mentioned above, hot is gradable (hot, hotter, hottest, or “very hot” or the like), but freezing, even though it can be substituted, without modifiers, for hot, is nongradable: “It’s really freezing” is a plausible informal comment, but it’s not a factual statement, and “It’s more freezing than it was earlier” is illogical.
Some adjectives are gradable or nongradable depending on meaning. For example, though you can refer to an elderly man who owns property as a very old landlord, it’s incorrect to use the phrase “very old landlord” to refer to a landlord you had a long time ago; the phrase “old landlord” cannot be intensified to convey a significant passage of time since the pertinent state of “landlord” (as in “my landlord”) existed.